Kaṭhopaniṣad 2.23-24—Self is Known thru Self Alone

“This Self cannot be known through much study, nor through the intellect, nor through much hearing. It can be known through the Self alone that the aspirant prays to; this Self of that seeker reveals Its true nature. One who has not desisted from bad conduct, whose senses are not under control, whose mind is not concentrated, whose mind is not free from anxiety about the result of concentration, cannot attain this Self through knowledge.“